Device and method for measuring microporous film on battery electrode plate, coater equipped with film measuring device, and coating method using film measuring method

ABSTRACT

There is provided a film measuring device capable of accurately and easily measuring the thickness of a microporous film formed on a battery electrode plate over the entire area of the film. A color CCD sensor  8  shoots the microporous film. A video board  11  converts a color tone of a color image signal obtained by the image pickup into gradation data of respective color components of RGB. After the data conversion, an image processing board  12  extracts line images of the respective color components. A calculator  14  obtains the thickness of the microporous film by referring to pre-measured film thickness reference values corresponding to the gradation data of the green or blue color component, which are stored in a table storage  13  as reference thickness table data, using the gradation data of the line image of the green color component or the blue color component as lookup data.

RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.11/507,619, filed on Aug. 22, 2006, now U.S. Pat. No. 7,679,739,claiming priority of Japanese Patent Application No. 2005-254333, filedon Sep. 2, 2005, the entire contents of each of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device and a method for measuring athickness and a weight per unit area of a microporous film formed on anelectrode plate for a battery, as well as a coater equipped with thefilm measuring device, and a coating method using the film measuringmethod.

2. Description of the Related Art

Heretofore, there has been used a technique of measuring the weight perunit area of a microporous film formed on an electrode plate for abattery, as disclosed in Japanese Laid-Open Patent Publication No. H8(1996)-96806 (D1), for instance. In D1, a β-ray emitter and a detectorare arranged as opposed to each other, with an electrode material beingfed in a certain direction between the β-ray emitter and the detector.The β-ray emitter and the detector are moved in cooperation with eachother in a width direction of a film to be measured, i.e. in a directionorthogonal to the feeding direction of the electrode material. Duringthe movements of the β-ray emitter and the detector, the β-ray emitteremits β-rays, and the detector detects the amount of the β-raystransmitted through the electrode material. The weight per unit area ofthe microporous film is measured by comparing a detection result on theβ-ray transmission with a reference transmission amount.

In the above arrangement, the measurement sites are only the positionswhere the β-ray emitter and the detector are moved, and accordingly, itis impossible to conduct the measurement for the entire area of theelectrode material. Also, it is hazardous to handle β-rays. Theinstallation condition for the β-ray emitter is constrained because aradioactive ray is used. Further, a qualified staff is required tooperate the β-ray emitter, which hinders usability of the β-ray emitter.In addition, the β-ray emitter is expensive. Furthermore, since themicroporous film is thin, a variation in β-ray transmission amount dueto a thickness variation of the microporous film is significantly small.Also, the variation in β-ray transmission amount is smaller than avariation in β-ray transmission amount due to thickness variations of ametal sheet as a base member, and an active material layer, which makesthe weight detection difficult. Further, even if a fluorescent X-ray isused in place of the β-ray, a variation in fluorescent X-raytransmission amount is significantly small, and an attenuation of thefluorescent X-ray in the air is significantly large, which also makesthe weight detection difficult.

Japanese Laid-Open Patent Publication No. H8 (1996)-309262 (D2) proposesa film thickness measuring device constructed such that: a UV ray isirradiated onto a surface of a golf ball coated with a clear coat madeof a clear coating material containing a fluorescent brightening agent;secondary emission rays obtained by the UV ray irradiation are capturedby a CCD camera; a contrast image is obtained by multilevel-processingthe acquired image data; and the thickness of the coat is measured basedon the contrast image.

It is, however, impossible to measure the film thickness, unless thefilm has a property responsive to a specific wavelength, even bymodifying the art recited in D2 so as to measure the thickness of thefilm formed on the film-like or sheet-like base member.

SUMMARY OF THE INVENTION

In view of the above problems residing in the conventional examples, itis an object of the present invention to provide a film measuring deviceand method capable of accurately and easily measuring a physical amountof a microporous film formed on an electrode plate of a secondarybattery over the entire area of the film, as well as a coater equippedwith the film measuring device, and a coating method using the filmmeasuring method.

An aspect of the present invention is directed to a film measuringdevice for measuring a physical amount of a microporous film formed onat least one of electrode plates of a positive electrode and a negativeelectrode of a secondary battery. The device includes: an image pickupsection for converting a color tone of a color image obtained bycapturing an image of the microporous film into gradation data ofrespective color components; a table storage for storing thereinpre-measured reference values of the physical amount of the microporousfilm corresponding to gradation levels of at least one of the colorcomponents in the form of a table; and a calculator for obtaining thephysical amount of the microporous film by referring to the referencevalues of the physical amount stored in the table storage, and by using,as lookup data, the gradation data of the at least one color componentamong the gradation data of the respective color components obtained bythe image pickup section.

Another aspect of the present invention is directed to a coaterincluding: the aforementioned film measuring device; a coating sectionfor coating a coating material for the microporous film after forming anactive material layer on a sheet-like base member; and a coating amountcontroller for controlling the coating amount for the microporous filmin the coating section in accordance with the physical amount of themicroporous film obtained by the calculator of the film measuringdevice.

Yet another aspect of the present invention is directed to a filmmeasuring method for measuring a physical amount of a microporous filmformed on at least one of electrode plates of a positive electrode and anegative electrode of a secondary battery. The method includes steps of:converting a color tone of a color image obtained by capturing an imageof the microporous film into gradation data of respective colorcomponents; and obtaining a physical amount of the microporous film byreferring to pre-measured reference values of the physical amount of themicroporous film corresponding to the gradation data of at least one ofthe color components obtained in the conversion step, using thegradation data of the at least one color component as lookup data, thereference values being stored as table data.

Still another aspect of the present invention is directed to a coatingmethod using the aforementioned film measuring method. The coatingmethod includes steps of: forming an active material layer on asheet-like base member; coating a coating material for the microporousfilm on the active material layer; and controlling the coating amountfor the microporous film in the coating step in accordance with thephysical amount of the microporous film obtained by the film measuringmethod.

These and other objects, features and advantages of the presentinvention will become more apparent upon reading the following detaileddescription along with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram showing an entire configuration of a coater equippedwith a film measuring device according to an embodiment of theinvention.

FIG. 2 is a graph showing relationships between thickness of an aluminalayer, and gradation levels of respective color components of red (R),green (G), and blue (B) in the case where the alumina layer is formed ona mixture layer formed by coating a carbon material on a copper foil.

FIG. 3 is a graph showing relationships between thickness of an aluminalayer, and gradation level of the color component (B) in the case wherethe alumina layer is formed on a mixture layer formed by coating acarbon material on a copper foil.

FIG. 4A is a graph showing relationships between thickness of blue ink,and gradation levels of respective color components of red (R), green(G), and blue (B) in the case where the blue ink is coated on a matteblack finished mixture layer.

FIG. 4B is a graph showing relationships between thickness of red ink,and gradation levels of respective color components of red (R), green(G), and blue (B) in the case where the red ink is coated on a matteblack finished mixture layer.

FIG. 4C is a graph showing relationships between thickness of green ink,and gradation levels of respective color components of red (R), green(G), and blue (B) in the case where the green ink is coated on a matteblack finished mixture layer.

FIG. 4D is a graph showing relationships between thickness of aqua colorink, and gradation levels of respective color components of red (R),green (G), and blue (B) in the case where the aqua color ink is coatedon a matte black finished mixture layer.

FIG. 4E is a graph showing relationships between thickness of pink colorink, and gradation levels of respective color components of red (R),green (G), and blue (B) in the case where the pink color ink is coatedon a matte black finished mixture layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a diagram showing an entire configuration of a coater 2 usinga film measuring device 1 according to a preferred embodiment of theinvention. The film measuring device 1 is adapted for measuring, asphysical amounts of a microporous film, the thickness and the weight perunit area of the microporous film formed on an electrode plate of alithium secondary battery. The lithium secondary battery includes apositive electrode containing a composite lithium oxide, a negativeelectrode containing a lithium retainable material, a separator, and anelectrolyte solution containing a non-aqueous solvent, wherein themicroporous film containing an inorganic oxide filler and a binder isadhesively formed on at least one of electrode plates of the positiveelectrode and the negative electrode. In this embodiment, an arrangementand an operation of the coater 2 for forming an alumina film on anegative electrode are illustrated and described as an example of theinvention.

The negative electrode 4 as an object under test has a sheet-like shape,and is spirally wound in the form of a roll 3. Hereinafter, the negativeelectrode 4 is called as “electrode sheet 4”. The electrode sheet 4 issequentially dispensed and fed through a plurality of guide rollers 5,so that an alumina paint is coated on the electrode sheet 4 by a coatingsection 6. After the coating, the electrode sheet 4 is guided to adrying furnace 7 where the paint coated on the electrode sheet 4 isdried. The film measuring device 1 is disposed at an outlet of thedrying furnace 7.

In the film measuring device 1, a color CCD sensor 8 and an illuminationlight source 9 are disposed on the side corresponding to the surface ofthe electrode sheet 4 where the alumina paint is coated. Immediately ata downstream of a position where the electrode sheet 4 comes out of thedrying furnace 7, the illumination light source 9 illuminates thealumina film on the electrode sheet 4, and the color CCD sensor 8sequentially captures images of the alumina film on an area over theentire width of the electrode sheet 4. The color CCD sensor 8 mayinclude plural sensing devices arrayed in a line in such a manner thatthe sensing devices capture images of the alumina film on an areasubstantially covering the entire width of the electrode sheet 4. Theillumination light source 9 is a straight tube-shaped fluorescent lampso that an area along the entire width of the electrode sheet 4 can beuniformly irradiated. In the case where ambient light of a sufficientlight amount with less external stray light is obtained at the imagecapturing positions of the color CCD sensor 8, the illumination lightsource 9 may be omitted.

A color image signal outputted from the color CCD sensor 8 is sent to animage controller 10. Then, a video board 11 in the image controller 10sequentially converts the input signal, i.e., a composite video signalobtained by superimposing a carrier chrominance signal to a luminancesignal into primary color signals of red (R), green (G), and blue (B) ofe.g. 8 bits, i.e. 256 gradation data. The color CCD sensor 8 and thevideo board 11 constitute an image pickup section.

An image processing board 12 in the image controller 10 extracts lineimages in the width direction of the electrode sheet 4 based on therespective primary color signals of RGB. It is effective to use thegreen color component or the blue color component in order to measurethe thickness of the alumina film. In view of this, a table storage 13stores therein pre-measured reference values of film thicknesscorresponding to the respective gradation levels of the green colorcomponent or the blue color component in the form of a referencethickness table. The effectiveness of the green color component or theblue color component will be described later.

A calculator 14 reads out a film thickness corresponding to a targetedgradation level by referring to the film thickness reference valuesstored in the table storage 13, and by using the gradation dataconcerning the green color component or the blue color component of therespective line images acquired by the image processing board 12, aslookup data. In the case where it is judged that there is no filmthickness data corresponding to the targeted gradation level in thetable storage 13, the calculator 14 obtains film thickness datacorresponding to the targeted gradation level, using various approachessuch as approximation or data interpolation for a film thicknesscharacteristic with respect to the targeted gradation level, accordingto needs. Comparison in the calculator 14 between the measured value andthe reference value in gradation level may be carried out in the unit ofpixels, or may be made by dividing the captured image into predeterminednumber of areas, and by calculating an average in each of the areas.

The calculator 14 judges whether the thickness of the alumina film lieswithin an allowable range relative to a predetermined reference value.If the calculator 14 judges that the film thickness lies out of theallowable range, the calculator 14 judges that the negative electrodeplate carrying the alumina film as a defective electrode plate. Then,the calculator 14 issues a marking signal to a marker 20 so that adefective area on the alumina film is identified. In this case, even ifthe film thickness lies out of the allowable range in the order ofseveral square centimeters (cm²) on an area of the negative electrodeplate, in an actual process, an area on the negative electrode in theorder of several square meters (m²) is required to be removed as adefective area. In view of this, the calculator 14 detects apredetermined area on the alumina film including the area where the filmthickness is judged to lies out of the allowable range, as a defectivearea.

After the test on the thickness of the alumina film is conducted, theelectrode sheet 4 is wound into a roll 15.

The table storage 13 stores therein data relating to the weight per unitarea of alumina, which is a material for the film, in relation to therespective gradation levels of the green color component or the bluecolor component in the form of a reference weight table, as well as thedata relating to the measured values of the film thickness correspondingto the respective gradation levels of the green color component or theblue color component. By the storage of the tables, the calculator 14 isallowed to calculate the thickness and the weight per unit area of thealumina film.

Also, the calculator 14 outputs a correction signal to a coating amountcontroller 21 so that the calculated film thickness coincides with apredetermined reference value. The coating amount controller 21 controlsthe coater 6 to make a coating amount constant by changing a coatingcondition in response to the correction signal. Specifically, in case ofusing a die coater, a rotation speed of a pump is changed to control thecoating amount. In case of using a gravure coater, a coating speed ratiois changed to control the coating amount. Thus, feedback control isperformed to attain a constant thickness for the alumina film.

An input section 22 is connected to the table storage 13. After the filmthickness of the electrode sheet 4 is measured, and the electrode sheet4 is withdrawn from the film forming process, an operator is allowed,using the input section 22, to input data obtained by actually measuringthe thickness and the weight per unit area of the alumina film forstorage into the table storage 13. In this way, by allowing the operatorto input the table data by way of the input section 22, a calibrationcurve concerning a relationship between gradation levels of therespective color components and the coating amount can be corrected, orthe number of sampling data can be increased. Thus, measurementprecision on the thickness and the weight per unit area of the film canbe enhanced.

An illuminance sensor 23 is disposed on the side of the electrode sheet4 opposite to the side where the color CCD sensor 8 and the illuminationlight source 9 are disposed. The illuminance sensor 23 measures theilluminance of the illumination light source 9 when transport of theelectrode sheet 4 is suspended, for instance, at the time of exchangingthe roll 3, 15. Alternatively, the illuminance sensor 23 may be disposedoutside of an end of the electrode sheet 4 so that the illuminance ofthe illumination light source 9 is constantly measured. In such a case,feedback control is performed so that an illumination controller 24controllably keeps the illuminance of the illumination light source 9constant based on a measurement result by the illuminance sensor 23.With this arrangement, the illuminance of illumination light can bemaintained constant, despite aging degradation of the illumination lightsource 9 or fluctuation of a power source voltage, thereby enablingaccurate film thickness measurement.

The above configuration realizes the coater 2 capable of accurately andeasily measuring the thickness and the weight per unit area of a filmcoated on the electrode sheet, and easily controlling the film formingprocess. Also, selecting at least one color component which is effectivein film thickness measurement enables to measure the thickness and theweight per unit area of a microporous film made of an intended material.Further, the thickness and the weight per unit area of the film over theentire area thereof can be measured by changing the image pickupposition, or by providing the image pickup position for the entire areaof the object under test.

The color component to be stored in the table storage 13 may be a colorcomponent that has a large contrast relative to the base member, and hasa relatively large change in gradation level in estimated variationranges for the thickness and the weight per unit area of the film. Amonochromatic color component may be used. Alternatively, combination ofplural color components may be used in the case where the plural colorcomponents show a remarkable change between plural regions within theaforementioned variation ranges. Further alternatively, the colorcomponent may be in the form of a composite signal obtained by combininga color-difference signal to a luminance signal, in place of the signalsrespectively representing the individual color components of RGB, orcyan (C), magenta (M), and yellow (Y).

FIG. 2 is a graph showing relationships between thickness of an aluminalayer, and gradation levels of the respective color components of RGB,in the case where the alumina layer is formed on a mixture layercontaining a carbon material on a copper foil.

Specifically, the alumina layer is formed by firstly coating a mixturelayer, as an active material layer, containing a carbon material on thecopper foil as a base member, and then coating an alumina paintcontaining an alumina as an inorganic oxide filler, and a binder on theactive material layer. In this case, the white powdery alumina paint iscoated onto the matte black finished mixture layer.

Referring to FIG. 2, first sampling data are denoted by referencenumerals βR1, βG1, and βB1; and second sampling data are denoted byreference numerals βR2, βG2, and βB2. Third sampling data denoted byreference numerals βR3, βG3, and βB3 are measurement data in which theviscosity of the alumina paint is changed by changing the compositionratio of the binder relative to the alumina. Specifically, an activematerial with a density of 1.63 g/cm³ is coated on a copper foil of 16μm in thickness so that the thickness of the active material layer onone surface of the copper film is 100 μm. Then, an alumina paint in NMP(N-methyl-2-pyrrolidone) solution, with a solid content ratio of 45% anda composition ratio of alumina to PVDF (polyvinylidene fluoride) at96:4, is coated on the active material layer.

As is obvious from FIG. 2, changes in gradation level among therespective color components of RGB are substantially the same. Thisshows that use of any color component among the three color componentsis acceptable. Also, measurement precision can be enhanced by combiningthe color components. FIG. 3 and Table 1 show film thickness withrespect to gradation level of the blue color component.

TABLE 1 on base member gradation of blue thickness (μm) color component0.3 97 1.3 128 1.7 133 2.5 134 3.0 135 3.7 138 4.0 140 4.2 143 4.5 1514.8 153 5.0 155 5.5 160 6.5 165

As shown in FIG. 3, it is possible to recognize a thickness variation inthe order of 1 μm, as a gradation level change in the gradation rangefrom 0 to 256. Titanium oxide and magnesia show substantially the samegradation level changes as the alumina used as the inorganic oxidefiller in the embodiment.

The film measuring device 1 in the embodiment not only measures thethickness and the weight per unit area of the alumina layer, but also iscapable of measuring a semitransparent film which is formed on anelectrode plate for a battery, and whose color tone is varied dependingon the film thickness.

FIGS. 4A, 4B, 4C, 4D, and 4E are graphs each showing relationshipsbetween ink thickness, and gradation levels of the respective colorcomponents of RGB in the case where ink of a certain color is coated ona matte black finished mixture layer. FIG. 4A shows a case that blue inkis coated, FIG. 4B shows a case that red ink is coated, FIG. 4C shows acase that green ink is coated, FIG. 4D shows a case that aqua color inkis coated, and FIG. 4E shows a case that pink color ink is coated.

The following is an analysis result on FIGS. 4A through 4E. Since athickness variation relative to the gradation level of the red colorcomponent is great in the case where the blue ink, the green ink, andthe aqua color ink are coated, it is comprehended that use of the redcolor component is effective for the blue ink, the green ink, and theaqua color ink. Since a thickness variation relative to the gradationlevel of the green color component is great in the case where the redink and the pink color ink are coated, it is comprehended that use ofthe green color component is effective for the red ink and the pinkcolor ink. The above clarifies that the thickness of a microporous filmformed on an electrode plate for a battery can be measured, using atleast color component of the captured color image.

The foregoing embodiment describes the film measuring device formeasuring the thickness and the weight per area of the microporous filmformed on the negative electrode plate of the lithium secondary battery.Alternatively, the invention is applicable to a film measuring devicefor measuring the thickness and the weight per area of a microporousfilm formed on at least one of electrode plates of a positive electrodeand a negative electrode in an alkaline secondary battery such as anickel-cadmium secondary battery, or a nickel-hydride secondary battery.

The following is a summary on the features of the invention.

A film measuring device according to a first aspect of the presentinvention is a device for measuring the thickness of a microporous filmin a lithium secondary battery including a positive electrode containinga composite lithium oxide, a negative electrode containing a lithiumretainable material, a separator, and an electrolyte solution containinga non-aqueous solvent, wherein the microporous film contains aninorganic oxide filler and a binder, and is adhesively formed on atleast one of electrode plates of the positive electrode and the negativeelectrode of the lithium secondary battery. The device includes: animage pickup section for converting a color tone of a color imageobtained by capturing an image of the microporous film into gradationdata of respective color components; a reference thickness table storagefor storing therein pre-measured reference values of the film thicknesscorresponding to gradation levels of at least one of the colorcomponents in the form of a table; and a calculator for obtaining athickness of the microporous film by referring to the film thicknessreference values stored in the reference thickness table storage, and byusing the gradation data of the at least one color component, as lookupdata, among the gradation data of the respective color componentsobtained by the image pickup section.

A film measuring device according to a second aspect of the presentinvention is a device for measuring the weight per unit of a microporousfilm in a lithium secondary battery including a positive electrodecontaining a composite lithium oxide, a negative electrode containing alithium retainable material, a separator, and an electrolyte solutioncontaining a non-aqueous solvent, wherein the microporous film containsan inorganic oxide filler and a binder, and is adhesively formed on atleast one of electrode plates of the positive electrode and the negativeelectrode of the lithium secondary battery. The device includes: animage pickup section for converting a color tone of a color imageobtained by capturing an image of the microporous film into gradationdata of respective color components; a reference weight table storagefor storing therein pre-measured reference values of the weight per unitarea corresponding to gradation levels of at least one of the colorcomponents in the form of a table; and a calculator for obtaining aweight per unit area of the microporous film by referring to the weightper unit area reference values stored in the reference weight tablestorage, and by using the gradation data of the at least one colorcomponent, as lookup data, among the gradation data of the respectivecolor components obtained by the image pickup section.

With the above arrangements, in the case where at least one of thepositive and negative electrodes in the lithium secondary battery has abattery electrode plate on which the microporous film containing theinorganic oxide filler and the binder is adhesively formed, in theprocess of forming the microporous film on the battery electrode plate,specifically, in measuring the thickness and the weight per unit area ofthe film in non-contact state using the image pickup section, the imagepickup section shoots the microporous film to obtain the color image,converts the color tone of the acquired color image into gradation dataof the respective color components, and outputs the gradation data.Also, the reference thickness table storage and the reference weighttable storage store therein the pre-measured thickness reference valuesand the pre-measured weight per unit area reference values as pluralsampling data, and the gradation data concerning one or more colorcomponents which is effective in measuring the thickness and the weightper unit area of the film corresponding to each other, in the form oftables, respectively.

The color component effective in measuring the thickness and the weightper unit area of the film is a color component that has a large contrastrelative to the base member, and has a relatively large change ingradation level in estimated variation ranges for the thickness and theweight per unit area of the film. A monochromatic color component may beused. Alternatively, combination of plural color components may be usedin the case where the plural color components show a remarkable changebetween plural regions within the aforementioned variation ranges.Further alternatively, the color component may be in the form of acomposite signal obtained by combining a color-difference signal to aluminance signal, in place of % the signals respectively representingthe individual color components of RGB, or cyan (C), magenta (M), andyellow (Y).

Also, the calculator for calculating the thickness and the weight perunit area of the film reads out the thickness and the weight per unitarea of the film corresponding to a targeted gradation level byreferring to the reference thickness table and the reference weighttable, and by using the gradation data of the at least one colorcomponent which is effective in measuring the thickness and the weightper unit area of the film among the gradation data of the respectivecolor components obtained by the image pickup section. If, however, thedata concerning the film thickness and the data concerning the weightper unit area corresponding to the targeted gradation level are notstored in the table storage, the calculator obtains data concerning thethickness and the weight per unit area of the film corresponding to thetargeted gradation level, using the techniques such as approximation ordata interpolation for the thickness and the weight per unit area of thefilm corresponding to the targeted gradation level, according to needs.

In the above arrangement, the thickness and the weight per unit area ofthe film can be accurately and easily measured. Also, selecting at leastone color component which is effective in measuring the thickness andthe weight per unit area of the film enables to measure the thicknessand the weight per unit area of a microporous film made of an intendedmaterial. Further, changing the image pickup position, or providing theimage pickup position over the entire area of the object under testenables to measure the thickness and the weight per unit area of thefilm over the entire area thereof.

In the film measuring device of the first aspect, preferably, themicroporous film is formed on the electrode plate of the negativeelectrode, the inorganic oxide filler is alumina, magnesia, or titaniumoxide, the lithium retainable material is a carbon material, the imagepickup section converts the color image into the gradation data of a redcolor component, a green color component, and a blue color component,the reference thickness table storage stores therein the film thicknessreference values corresponding to the gradation levels of the greencolor component and the blue color component, and the calculator usesthe gradation data of at least one of the green color component and theblue color component as the lookup data.

With the above arrangement, in the case where the inorganic oxide filleras the material of the microporous film formed on the negative electrodeis the alumina, magnesia, or titanium oxide, a white powdery coat iscoated on a mixture layer. In the case where the lithium retainablematerial is the carbon material, a matte black finished mixture layer isformed. When the image pickup section captures the image of the whitepowdery coat coated on the matte black finished mixture layer in termsof the color components of RGB, changes in gradation level concerningthe green color component and the blue color component is great relativeto changes in the film thickness.

In view of the above, by way of the reference thickness table storageand the calculator, obtaining the thickness of the microporous film,using the at least one of the green color component and the blue colorcomponent leads to optimal measurements of the thickness of themicroporous film formed on the negative electrode of the lithiumsecondary battery.

Preferably, each of the film measuring devices further includes anillumination light source for projecting illumination light onto themicroporous film; an illuminance sensor for detecting an illuminance ofthe illumination light; and an illumination controller for feedbackcontrolling the illumination light source so that the illuminance of theillumination light is made constant in accordance with the illuminancedetected by the illuminance sensor.

With the above arrangement, the illuminance sensor detects theilluminance of the illumination light, which may be changed due to agingdegradation of the illumination light source, or fluctuation of a powersource voltage, and the illumination controller feedback controls theillumination light source so that the illuminance of the illuminationlight is made constant in accordance with the detected illuminance.

The above arrangement enables to maintain the illuminance of theillumination light constant, despite the aging degradation of theillumination light source or the fluctuation of the power sourcevoltage, thereby allowing accurate measurements of the thickness and theweight per unit area of the film.

Preferably, the film measuring device of the first aspect furtherincludes an input section for allowing an operator to input actuallymeasured film thicknesses as the reference values corresponding to thegradation levels of the respective color components for storage into thereference thickness table storage.

With the above arrangement, allowing the operator to withdraw thebattery electrode plate after the image pickup, and to input theactually measured film thickness data into the reference thickness tablestorage storing the pre-measured film thicknesses as the referencevalues enables to correct a calibration curve concerning a relationshipbetween gradation levels of the respective color components, and coatingamount, and to increase the number of the sampling data to therebyenhance measurement precision of the film thickness.

A coater according to another aspect of the present invention includes:the film measuring device having the image capturing section, thereference thickness table storage, and the calculator; a coating sectionfor coating a coating material containing the inorganic oxide filler andthe binder after forming an active material layer on a sheet-like basemember; a coating amount controller for controlling a coating amount ofthe coating material in the coating section in accordance with thethickness of the microporous film obtained by the calculator.

With the above arrangement, the coating amount can be automaticallycontrolled in accordance with the film thickness measured by the filmmeasuring device.

Preferably, in the coater, the calculator judges whether the obtainedthickness of the microporous film lies within an allowable rangerelative to a predetermined reference value, and detects a predeterminedarea on the microporous film including an area where the thickness ofthe microporous film is judged to lie out of the allowable range, as adefective area.

With the above arrangement, it is easy to detect the predetermined areaon the microporous film including the area where the thickness of themicroporous film is judged to lie out of the allowable range, as thedefective area, which is appropriate for the actual process, as comparedwith a case that merely the area where the thickness of the microporousfilm is judged to transgress the allowable range is detected as thedefective area.

As described above, the invention is advantageous in accurately andeasily measuring the thickness and the weight per unit area of amicroporous film made of an intended material over the entire area ofthe film, automatically controlling the coating amount of the coatingmaterial for the film in accordance with the measured thickness and themeasured weight per unit area of the film, and easily detecting thedefective area of the film in a process of forming the film on anelectrode plate of a secondary battery.

This application is based on Japanese Patent Application No. 2005-254333filed on Sep. 2, 2005, the contents of which are hereby incorporated byreference.

1. A film measuring method for measuring a physical amount of amicroporous film to be formed on at least one of electrode plates of apositive electrode and a negative electrode of a secondary battery, themethod comprising steps of: converting a color tone of a color imageobtained by shooting the microporous film into gradation data ofrespective color components; and obtaining a physical amount of themicroporous film by referring to pre-measured reference values of thephysical amount of the microporous film corresponding to the gradationdata of at least one of the color components obtained in the conversionstep, using the gradation data of the at least one color component aslookup data, the reference values being stored as table data, wherein inthe conversion step, the color image is converted into the gradationdata of at least one color component selected in accordance with amaterial of the microporous film, and the table data represents thereference values of the physical amount of the microporous film storedcorresponding to gradation levels of the at least one color component.2. The film measuring method according to claim 1, wherein the secondarybattery includes a positive electrode containing a composite lithiumoxide, a negative electrode containing a lithium retainable material, aseparator, and an electrolyte solution containing a non-aqueous solvent,and the microporous film is adhesively formed on the electrode plate,and contains an inorganic oxide filler and a binder.
 3. A film measuringmethod for measuring a physical amount of a microporous film to beformed on at least one of electrode plates of a positive electrode and anegative electrode of a secondary battery, the method comprising of:converting a color tone of a color image obtained by shooting themicroporous film into gradation data of respective color components; andobtaining a physical amount of the microporous film by referring topre-measured reference values of the physical amount of the microporousfilm corresponding to the gradation data of at least one of the colorcomponents obtained in the conversion step, using the gradation data ofthe at least one color component as lookup data, the reference valuesbeing stored as table data, wherein the secondary battery includes apositive electrode containing a composite lithium oxide, a negativeelectrode containing a lithium retainable material, a separator, and anelectrolyte solution containing a non-aqueous solvent, the microporousfilm is adhesively formed on the electrode plate of the negativeelectrode, and contains an inorganic oxide filler and a binder, theinorganic oxide filler is alumina, magnesia, or titanium oxide, thelithium retainable material is a carbon material, in the conversionstep, the color image is converted into the gradation data of a redcolor component, a green color component, and a blue color component,the table data represents the reference values of the physical amount ofthe microporous film stored corresponding to gradation levels of thegreen color component and the blue color component, and the lookup datais the gradation data of at least one of the green color component andthe blue color component.
 4. The film measuring method according toclaim 1, wherein the physical amount of the microporous film is athickness of the film, and the table data is reference thickness tabledata representing the pre-measured reference values of the filmthickness stored corresponding to gradation levels of the respectivecolor components.
 5. The film measuring method according to claim 1,wherein the physical amount of the microporous film is a weight per unitarea of the film, and the table data is reference weight table datarepresenting the pre-measured reference values of the weight per unitarea of the film stored corresponding to gradation levels of therespective color components.
 6. A coating method using the filmmeasuring method of claim 1, comprising steps of: forming an activematerial layer on a sheet-like base member; coating a coating materialfor the microporous film on the active material layer; and controllingthe coating amount for the microporous film in the coating step inaccordance with the physical amount of the microporous film obtained bythe film measuring method.
 7. The coating method according to claim 6,further comprising steps of: judging whether the physical amount of themicroporous film obtained by the film measuring method lies within anallowable range relative to a predetermined reference value; anddetecting a predetermined area on the microporous film including an areawhere the physical amount of the microporous film is judged to lie outof the allowable range, as a defective area.
 8. The film measuringmethod according to claim 1, wherein in the conversion step, the colorimage is converted into the gradation data of at least one of a redcolor component, a green color component and a blue color componentselected in accordance with the material of the microporous film, thetable data represents the reference values of the physical amount of themicroporous film stored corresponding to gradation levels of the atleast one of the red color component, the green color component and theblue color component.